AN O-18-SIMS STUDY OF OXIDE-GROWTH ON NICKEL MODIFIED WITH CE IMPLANTS AND CEO2 COATINGS

The influence of surface applied reactive elements on the mechanism of the early stages of oxidation of Ni at 973 and 1073 K was studied by the O-18-SIMS technique. Polycrystalline Ni was implanted with Ce ions as well as coated with CeO2 thin films by both reactive sputtering and the sol-gel method. In addition, sol-gel coatings were applied to the surface of (100) and (111) Ni single crystals. It has been found that the oxide growth mechanism depended on the surface structure of the Ni substrate before modification with Ce/CeO2. For polycrystalline Ni, the decisive factor was surface finish. For oxides grown on chemically polished Ni (both CeO2 sputter-coated and Ce ion-implanted), a higher contribution of inward oxygen diffusion to the growth process was found compared with the faster growing scales on CeO2 sol-coated Ni with a surface finished by mechanical polishing. For Ni single crystals the suppression of outward diffusion of Ni2+ due to the presence of the reactive element depended on the crystallographic orientation of the modified surface. CeO2 sol-coatings on (111) Ni resulted in oxide growth by a higher contribution of inward oxygen diffusion than that observed on CeO2-coated (100) Ni.